1. Field of the Invention
The present invention relates to exhaust gas treatment and in particular to circulating dry scrubber (CDS) systems and methods utilizing sorbent particles to treat exhaust gases for pollution reduction.
2. Description of the Related Art
CDS technology has many advantages over other FGD systems such as limestone wet FGD and lime-based spray drying absorption (SDA). Among the most appealing benefits of CDS are: low capital costs, small footprint, simple construction with few moving parts, carbon steel construction, the absence of a liquid blow-down stream to be treated, and the production of a dry spent sorbent product. However, limestone wet FGD systems continue to dominate the emissions control marketplace due to their inherently low operating costs. This is primarily due to the efficient use of an inexpensive limestone feedstock. Limestone scrubbers typically are able to achieve greater than 90% sulfur capture with a stoichiometric ratio (moles calcium carbonate fed per moles sulfur dioxide captured) approaching one.
The concept of circulating dry scrubbing is well known in the art. See, for example, Neathery, J. K., “A Fundamental Study of Circulating Bed Absorption for Flue Gas Desulfurization”, Ph.D. Dissertation, University of Kentucky, 1993. CDS technology incorporates dry sorbent recirculation in a lean-phase transport reactor to achieve sulfur capture. Hydrated lime and humidification water are injected into the bottom of a reaction chamber concurrently with flue gas. The flue gas suspends, dries, and transports the sorbent through the reaction vessel and out into a particulate collector. To maintain suspension in the reaction vessel under varying loads from the furnace, a large portion of both the spent and unutilized sorbent streams are recycled into the reactor vessel as a dry powder. The recycle of sorbent, from both within the reactor and via the particulate control device, improves the sorbent utilization over other semi-dry methods such as spray drying absorption (SDA). However, since the flue gas is to remain several degrees above the wet bulb or saturation temperature, the liquid phase coverage of the recycled solids quickly evaporates due to the excellent mass transfer and the abundant surface available in the riser section.
Although CDS is called a “dry” scrubbing process, it is water, either adsorbed or sprayed onto the sorbent material, that is the reactive phase accomplishing SO2 capture with lime to form a CaSO3/CaSO4 reaction product. Fresh hydrated lime and recycled sorbent particles capture a portion of the water droplets from humidification spray nozzles by a combination of inertial impaction and interception. The internal reflux rate of these internally recycled particles can be of the same order as the fine particulate recycled externally from a bag house or electrostatic precipitator (ESP). If the resulting sorbent slurry were to completely cover every CDS particle equally, the thickness of the slurry layer would range from 0.1 to 0.6 μm, depending on the volumetric rate of humidification water (i.e., the approach-to-saturation temperature or AST) and the reactor solids concentration.
As the wetted area of each particle begins to evaporate, SO2 is absorbed and reacts with the dissolved Ca(OH)2 to form mostly CaSO3.H2O, Since CaSO3 is relatively insoluble under these conditions, fine crystals will precipitate in solution and tend to block or cover unused hydrate surface area. When the water phase completely evaporates, the SO2 reaction is nearly halted. Previous experiences with measuring the evaporation rate within the riser section have shown that this bulk liquid phase can evaporate in a time on the order of one second. One could easily improve the utilization of the lime sorbent by adding additional humidification spray water. However, as the approach-to-saturation temperature is lowered, the flow properties of the recycle material can be compromised. In addition there can be issues of the removing particulate from either a baghouse or ESP device that can be encountered. Deposition of solids within the riser and within the recycle conveying system is also an operational concern the lower is the approach-to-saturation temperature.
Previous works on improving sorbent utilization for dry scrubbing technologies have focused on several areas including: optimization of the initial surface area of the virgin lime hydrate with additives including fly ash; suppressing the water vapor pressure of the liquid phase by dissolved salts from plant waste streams to slow water phase evaporation thereby extending the desulfurization reaction; and creating lime/silicate ash sorbents.
Optimization of Initial Lime Hydrate Surface Area.
It has been shown that superior surface area can be obtained using the reaction of hydrated lime with coal fly ash mixtures. See, e.g., Jozewicz, W., Jorgensen, C., Chang, J. C. S., Sedman, C. B. and Brna, T. (1988a) Development and pilot plant evaluation of silica-enhanced lime sorbents for dry flue gas desulfurization. J.A.P.C.A. 38, 796-805. Incorporating this feature into dry lime slaking systems would be problematic since the advantages of the dry method would be negated. In order to take advantage of the formation of high surface area calcium silicate/aluminates solids, a wet slurry system with sufficient hold-up to allow for sufficient reaction times would need to be implemented. Many of the experimental results for these sorbents were obtained with very long reaction times (>16 hours); however, a sufficient amount of increased surface area can be realized in less than 30 minutes.
Reactivation of Lime in the External Sorbent Recycle Stream.
The resulting unreacted calcium sorbent in the CDS recycle stream is due in large part to CaSO3 filling of mesopores and blocking pathways to fresh sorbent surfaces. The addition of water to the ground mixture may also provide additional activation by allowing for diffusion of solubilized calcium hydrate. The recycle flow rate is on the order of 100 times that of the fresh sorbent. Consequently, treating or reactivating the entire recycle stream is not practical, especially if the activation method includes water. However, it has been proposed that, if only a 1-2% of the recycle stream is sufficiently reactivated and rehydrated, then the sorbent ratio could be decreased dramatically. Ash present in the recycle stream could promote the formation of high surface area calcium silicate/aluminate in the reactivated sorbent. See, e.g., Liu, Chiung-Fang, Shin-Min Shih, and Ren-Bin Lin, “Effect of Ca(OH)2/fly ash weight ratio on the kinetics of the reaction of Ca(OH)2/fly ash sorbents with SO2 at low temperatures,” Chemical Engineering Science 59 (2004) 4653-4655 and Garea, A., J. R. Viguri and A. Irabien, “Kinetics of flue gas desulphurization at low temperatures: fly ash/calcium (3/l) sorbent behavior,” Chemical Engineering Science, Vol. 52, No. 5, pp. 715-732, 1997. See also Ren-Bin Lin, Shin-Min Shih, and Chiung-Fang Liu, “Structural Properties and Reactivities of Ca(OH)2/Fly Ash Sorbents for Flue Gas Desulfurization,” Ind. Eng. Chem. Res. 2003, 42, 1350-1356.
Suppressing the Water Vapor Pressure by Additives and Extending the Presence of a Reactive Liquid Phase.
The addition of deliquescent salts into CDS systems has been shown to effectively increase sorbent utilization. See, e.g., Ruiz-Alsop, R., G. Rochelle, “Effect of deliquescent salt additives on the reaction of SO2 with dry Ca(OH),” ACS Symp. Ser. 319 (1986) 208. However, the equilibrium moisture content of the spent solids is much higher as a result. Consequently, problems related to solids deposition and difficulties in cleaning filter cakes from fabric filtration devices may be an issue if the salts are over-fed. Additionally, additives such as calcium chloride will add to operating costs. One economical option to consider for vapor pressure depression is using dissolved salts/solids from plant waste streams such as cooling tower and wet FGD blow-down stream for humidification water.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for systems and methods that allow for improved sorbent utilization in CDS systems. There also remains a need in the art for such systems and methods that are easy to make and use. The present invention provides a solution for these problems.